Well, on a single-engine craft, it also reduces torque/P-factor/etc. effects.
I think that award belongs to the Republic XF-84H. Three-bladed prop, twelve-foot diameter, 3,000 rpm. Do the math.
The world record for a propeller driven aircraft was set in 1973 outside Las Vegas in a big ol’ fat assed 1944 Grumman Bearcat - modified for spped mind you with various bits of other aircraft which kinda bolted on to an acceptable degree.
The speed? 528 miles per hour. The blade count? 4
At those speeds the pitch on the blades is incredibly steep, and supersnoic tip efficiency is already being thresholded due to forward velocity plus rotational velocity. And there’s the rub - the faster you go forward, the more of your blade is going supersonic whether you like it or not - due to maximum rpm plus forward velocity.
It’s generally accepted that the speed record of 528mph is probably going to stand the test of time. Even “The Galloping Ghost” with all it’s wonderful aerodynamic innovations had a limit of around 500mph. The airframe itself was probably capable of going supersonic, possibly, but the blade inefficiency of high rpm plus forward velocity means the prop just keeps going further and further into supersonic territory on an exponential basis for each extra mph gained.
Interestingly, a Grumman Bearcat was about as aerodynamic as a brick - and that’s being kind to the bricks of this world. Those Wright Cyclone engines were almost 5 feet in diameter. I think the plane in question changed the original Pratt and Whitney twin wasp in favour of a Wright donk. One would think that the slim inline water cooled V12’s of this world would have ruled the outright speed war, but the record books are adamant - a Bearcat is the all time champ.
At least according to Wikipedia, the Tupolev Tu-95 is also in the running. 4 twin props, each of them supersonic.
Wikipedia mentions that just one media source (YouTube) states that the blade tips of the Tu-95 operate at supersonic speeds. I don’t think this is correct since it would make the props very ineffecient. If you look at videos of the Tu-95 flying, you can virtually see the blades turning, which means the gearboxes have a very high reduction ratio.
Also, it was mentioned here that the B-36 had six counter-rotating props. It had six large regular three-blade props. The XB-35 had 4 four-bladed counter-rotating props which were soon changed to larger single rotation props.
I did a project quite a while back designing and building a remotely controlled submersible. One of my teams efforts was the propeller design. What you say is true for water propellers, the optimal design is a single blade, but you can’t balance it, so 2 is the ideal compromise. This has to do with efficiency of transmitting motor (we used electric motors) power to the water via props (screws) and trying to minimize the interference of one blade on another.
There is also the power to weight ratio to consider, if you have a lot of power you may consider adding more blades, this decreases efficiency but allows more power to be transmitted. In watercraft this is good for high power - low speed applications such as a tug boat or the submersible we built (4 blade x 2 counter-rotating main drive).
Also like the supersonic issue in air props, there is a like issue with water props (screws) called cavitation which limits the speed that a screw can be rotated, the design efficiency (there are anti-cavitation props that can spin faster then standard designed props but deliver less efficiency of power transfer).
One of the engineering books stated that proper screw selection was as much as a art form and a engineering problem.
An ATR72-500 with 6 bladed props. Not noticeably noisier at ground speeds than four bladed props on smaller sized Friendship F27 and apparently quieter in flight.
This makes sense, and is something that’s been at the back of my mind. As stated before, compromises must be made. It sounds like the balance turns to a greater number of blades when power is greater. The efficiency of the blades decreases, but they make better use of the increased power; so the scales tip toward more blades.
I still wonder about marginal systems. I’ve mentioned the Cessna 182 Skylane, which uses a nominally 230 hp engine. It looks like there was a transition period in the mid-'80s where the C182R went from two-bladed props to three-bladed. Some older models have been retrofitted, but for 30 years two blades was enough. The Cessna 177 Cardinal (1968-1978) used two-bladed props. (NB: The 1968 model had a 150 hp engine, and was underpowered. In 1969 the C177B introduced the 180 hp engine, and the retractable-gear version – 177RG – introduced in 1971 had a 200 hp engine.) I’ve seen some 180 hp and 200 hp versions with three-bladed props retrofitted.
I’m guessing that for certain applications (e.g., climbing) a three-bladed prop is warranted on lower-power engines?
I would guess that for certain application (e.g., low performance aircraft with low power engines) it doesn’t matter a whole lot, and the props changed because Cessna or whichever got a better deal on props from a different maker.
In the homebuilt airplane community, it is generally found that a two blade propeller is faster, but three blades climb better ( all things being equal, which they never are).
In the lightplane regime, IIRC 2 vs 3 blades is/was mostly a marketing /styling decision. A 3-blade prop hub is about 50% more complex in terms of parts count, with a corresponding cost increment.
For roughly 60-200Hp you can swing a 2-blade of reasonable span & absorb all the power. At roughly 300Hp and above, you really need 3 blades to absorb all the power unless they’re real long, which would drive the rest of the aircraft configuration too much.
Somewhere in the 200-350hp range there’s a turnover point.
At the other end of the scale … With the likely re-emergence of open rotor designs for big aircraft propulsion, we’ll be looking at things with 2 rotors of between 8 & 14 blades each. That’s a 16-28 blade “prop” to absorb all the torque the core is producing.
I suspected this. Three-bladed props just look cooler, and give the impression of more power. I think that when they’re charging half a million simoleons for a four-seat airplane, buyers expect a little style.
I was unaware of that. Is there an advantage to open rotors, vs. a turbofan?
Why would a prop that pulls you upward faster, pull you in level flight slower?
I think he means greater performance in climbing, less performance in level flight, 2 vs. 3 in each category, not that level flight is slower than climbing.
Ref Johnny L.A. …
Open rotors are expected to be 10-15% more fuel efficient than the very best turbofans just now coming out - CFM’s Leap 1 & Pratt’s GTF series. Which are themselves 2-8% better than current top-of-line turbofans.
Open rotor’s fuel burn will be a total game-changer for airliner economics. If it works.
The four big flies in the ointment are 1) noise / vibration, 2) rotor burst / blade loss vs. the existing certification standards, 3) operations in icing conditions, and 4) passenger perception.
Much of the reason both Airbus & Boeing just punted on replacing the A320 & 737 series and went with a quicky reengine job for the A320 NEO & 737MAX projects is that it’s just a few years too early to know whether open rotor will pan out or not. The airframe integration is very dfferent for the two engine types. If either manufacturer bet now on latest-generation-plus-1 turbofans for a clean-sheet design and 3 or 4 years from now open rotors become obviously practical, they’d have just wasted several billion dollars.
So they’re waiting to see what does or doesn’t work.
Unfortunately, wiki is pretty much silent on the latest open rotor research.
I wasn’t very clear. You test the same plane, once with a two bladed propeller, and once with a three bladed propeller. I understood the claim to be that the three bladed test will climb faster than the two bladed test, but will be slower in level flight than the two bladed test.
Is that the claim?
I don’t understand how that would work, given it’s all a function of throwing air backwards, level flight or not.
This is an issue with fixed pitch propellors which are commonly used on homebuilts.
It’s essentially like a car with a one-speed transmission. You can set the gearing for good acceleration or for good cruising economy or for max top speed. But you are stuck with the tradeoff.
In general in an airplane you don’t cruise by setting a speed and adjusting engine power to acheive it. You set engine power to a particular output and accept whatever speed that gives you today. Typical output settings are max continuous, or best economy, or one of a couple canned compromise output settings between those extremes.
In the specific case of climb versus cruise prop performance, the driving issue is the difference in airspeed. How effectively any given prop converts crank torque into thrust depends on blade count, blade angle and speed of the ambient flow coming into the prop (otherwise known as the airspeed of the airplane it’s attached to).
So a prop optimized to couple power effectively at climb airspeeds will become less efficient when operated at faster cruise airspeeds. And vice versa.
Would this be equally true of a two or a three bladed prop? I refer you to my last post where I tried to clarify my question. It may be that I misunderstood the post made ThisOneGuy.
IMO (we’re getting out past the edge of my quality knowledge and into intuition and “everybody knows …”-type factoids) …
The 2- or 3-ness is effectively a side issue. IOW, that’s not what’s actually *driving *the differing behavior. The homebuilders are not able to buy props which are identical in all parameters except blade count. The 3-blade for a given engine will have a shorter span = smaller diameter. And a different washout profile AKA the shape of the change in pitch between the root and the tip. And perhaps even a different airfoil cross section.
As a general matter I can readily see the engine & prop manufacturers working together to deliver a 3-blade better optimized for climb and a 2-blade better optimized for cruise. But that’s a matter of cooperative engineering and marketing, not a matter of the physics delivering a hard constraint. They *could *build a 3-blade which is bettter for cruise and a 2-blade which is bettter for climb.
I’m betting that for the speeds & power levels we’re talking about the engineers have found that the best 3-blade climb prop is better than the best 2-blade climb prop. And the best 2-blade cruise prop is bettter than the best 3-blade cruise prop.
So while the homebuilders are accurately reporting their performance findings, they’re seeing correlation, not causation.
Pretty much unrelated but I’d rather ask here then start a new thread. I’m only vaguely aware of propellers that are not fixed pitch, how common are they? How do the mechanics of these things work. Can they adjust enough to put the plane in reverse(on the ground)